A multi-stage gearshift mechanism of an automatic transmission includes a plurality of planetary gearsets. A powertrain having such a plurality of planetary gearsets varies the torque in multi-stages and outputs it to an output shaft when receiving a converted engine torque from a torque converter. The more speeds the powertrain of an automatic transmission has, the better power performance and fuel consumption. Therefore, it is desirable to have as many speeds as possible in powertrains.
Even for the same number of speeds, durability, power transmission efficiency, and size/weight of a transmission are substantially dependent on how planetary gearsets are arranged. Therefore, research for more structural strength, less power loss, and more compact packaging are under continuing investigation.
Usually, development of a powertrain using planetary gearsets does not devise a wholly new type of planetary gearsets. To the contrary, it invokes how single/double pinion planetary gearsets are combined, and how clutches, brakes, and one-way clutches are disposed to the combination of planetary gearsets such that required shift speeds and speed ratios are realized with minimal power loss.
As for a manual transmission, too many speeds cause a driver the inconvenience of excessive manually shifting. However, for an automatic transmission, a transmission control unit automatically executes shifting by controlling the operation of the power train, and therefore, more speeds usually implies more merits.
Accordingly, research of four-speed and five-speed powertrains has been undertaken, and recently, a powertrain of an automatic transmission enabling six forward speeds and one reverse speed has been developed.
However, almost all the clutches and brakes of such powertrains are disposed to a rear of the transmission making it difficult to design hydraulic lines for operation of the powertrain. Furthermore, such powertrains can only provide a small range of speed ratios for a reverse speed, thereby deteriorating hill climbing ability in a reverse range. A step ratio between speed ratios of the second and third speeds is large, and accordingly, a high tip-in shock occurs when driving at low speeds in urban areas. Also, the speed ratio of the forward first speed is large, thereby frequently causing tire slip when accelerating the vehicle from a standstill position. Further, the speed ratio in the highest speed is low, and accordingly, driving torque is lowered in the highest speed.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known in this country to a person of ordinary skill in the art.